linux-kernel - Re: [6.2][regression] after commit ffcb754584603adf7039d7972564fbf6febdc542 all sound devices disappeared (due BUG at mm/page

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Message-ID: <f77a41bd-9df7-87c0-9b3c-e9e1a5deaa63@arm.com>
Date:   Fri, 16 Dec 2022 12:52:56 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>,
        Takashi Iwai <tiwai@...e.de>, alsa-devel@...a-project.org,
        m.szyprowski@...sung.com,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        iommu@...ts.linux.dev
Subject: Re: [6.2][regression] after commit
 ffcb754584603adf7039d7972564fbf6febdc542 all sound devices disappeared (due
 BUG at mm/page_alloc.c:3592!)

On 2022-12-16 12:15, Christoph Hellwig wrote:
> On Fri, Dec 16, 2022 at 11:40:57AM +0000, Robin Murphy wrote:
>> On 2022-12-16 06:46, Christoph Hellwig wrote:
>>> Ok, it seems like the sound noncontig alloc code that I already
>>> commented on as potentially bogus GFP_GOMP mapping trips this.  I think
>>> for now the right thing would be to revert the hunk in dma-iommu.c
>>> (see patch below).  The other thing to try would be to remove both
>>> uses GFP_COMP in sound/core/memalloc.c, which should have the same
>>> effect.
>>
>> Or we explicitly strip the flag in dma_alloc_noncontiguous() (and maybe
>> dma_alloc_pages() as well) for consistency with dma_alloc_attrs(). That
>> seems like it might be the most robust option.
> 
> In the long run warning there and returning an error seems like the
> right thing to do, yes. I'm just a little worried doing this right now
> after the merge window.

Fair point, I guess nobody else actually implements 
dma_alloc_noncontiguous(), and dma_alloc_pages() seems a bit of a grey 
area since it is more of an explicit page allocator. So yeah, just 
restoring iommu-dma (perhaps with a mild VM_WARN_ON?) seems like a 
sufficiently safe and sensible fix for the short term. You can have my 
pre-emptive ack for that.

Cheers,
Robin.